Exhaust gas from internal combustion engines, power plants, industrial furnaces, heaters, diesel engines, and other devices contains nitrogen oxides, carbon monoxide, and unburned hydrocarbons. Exhaust gases from these sources also contain excess levels of oxygen, water vapor, and sulfur dioxide.
Emissions of nitrogen oxides, carbon monoxide, and hydrocarbons are subject to limits due to environmental regulations.
Nitrogen oxides from stationary and mobile sources are one of the causes of acid rain. Various methods have been proposed to reduce the nitrogen oxide emissions from exhaust gases.
Nitrogen oxides (NO2 and NO, hereafter collectively referred to as NOx) in exhaust gas from gasoline engines are normally removed by using three-way catalysts. Three-way catalysts are not effective in removing NOx from exhaust gas having high concentrations of oxygen, such as the exhaust gas from gas turbines, diesel engines, and gasoline engines operated in a lean burn mode, because there is not sufficient reducing agent in the exhaust gas to reduce the NOx.
Typically, the exhaust gas from combustion devices that produce effluent gas containing excess oxygen is in the temperature range of 300 to 600° C. The exhaust gas contains oxygen, water vapor, small amounts of SO2, carbon monoxide, unburned hydrocarbons, nitrogen, and NOx. Selective reduction of NOx in this oxidizing environment is challenging.
The existing technologies for selectively reducing NOx in exhaust gas streams that contain excess oxygen cannot meet the future stringent emission standards. This has prompted intensive and extensive R&D activities for improved lean-NOx reduction technology.
Nitrogen oxides from large, stationary combustion sources such as power plants can be catalytically reduced with ammonia through the process of ammonia selective catalytic reduction (SCR).
Ammonia is toxic, however. The levels of ammonia that are introduced into the gas stream must be carefully controlled to avoid emitting excess ammonia into the atmosphere. Further, use of ammonia SCR generally requires large equipment, because the reactors and the control equipment in ammonia SCR applications are complex. Users of ammonia are also required to obtain special permits from local and federal authorities for the transportation, proper delivery and use of ammonia. Applications of ammonia SCR are therefore generally limited to large facilities such as power plants.
Nitrogen oxides can be reduced non-catalytically with reducing agents such as ammonia, hydrogen, carbon monoxide, or hydrocarbons. Because no catalyst is used, the reducing agent must be added in a larger amount than stoichiometric relative to the oxygen in the exhaust gas in order to effectively remove the NOx. The non-catalytic method is therefore limited to exhaust gas having low oxygen levels. Few exhaust streams contain such low levels of oxygen.
Therefore, many attempts have been made to develop catalysts and methods that can reduce NOx in exhaust gas containing high oxygen content, water vapor, and sulfur dioxide by either using hydrocarbons that are present in the exhaust gas and/or by injecting hydrocarbons or alcohols into the exhaust gas to achieve NOx reduction in lean atmospheres.
Catalysts containing zeolites, with or without transition metals, have been used to selectively reduce NOx with hydrocarbons. Some references include: Iwamoto et al. (Applied Catalysis 70, L15 (1991)); Held et al. (Society of Automotive Engineers (SAE) Technical Paper, Ser. No. 900496 (1990)); Takeshina et al. (U.S. Pat. No. 5,017,538); U.S. Pat. No. 5,260,043 to Armor et al.; Hamon et al. (U.S. Pat. No. 6,063,351); J. N. Armor, (Catalysis Today 26, 147 (1995)); Hall et al. (U.S. Pat. No. 6,033,641); Feng and Hall, Journal of Catalysis, 166, 368 (1997); U.S. Pat. No. 6,645,448 to Cho et al.; Subbiah et al. (Allied Catal. B: Environmental, 42, 155 (2003); Li and Flytzani-Stephanopoulos (Journal of Catalysis 182, 313 (1999) and Applied Catalysis B: Environmental 22, 35 (1999)); A. P. Walker, Catalysis Today 26, 107 (1995b); J. N. Armor, Catalysis Today 26, 99 (1995); Traa et al. Microporous and Mesoporous Materials 30, 3 (1999); and J. N. Armor, Catalysis Today 31, 191 (1996). There is general agreement that, although zeolite-based formulations are promising, more work needs to be done to identify stable and durable catalysts that can selectively reduce NOx under lean conditions.
In contrast to the mixed results obtained for zeolite-based catalysts, there has been significant progress on non-zeolite based formulations for selective catalytic reduction of NOx using hydrocarbons under lean conditions, particularly for silver on alumina-type catalysts.
Some representative silver on alumina NOx reduction catalyst references include; Yoshida et al. (U.S. Pat. No. 5,714,432); Yoshida et al. (U.S. Pat. No. 5,534,237); Itoh et al. (U.S. Pat. No. 5,559,072); Kharas (U.S. Pat. No. 5,980,844); Yu et al., Applied Catalysis B: Environmental 49, 159, (2004); Bogdanchikova et al., Applied Catalysis B: Environmental 36, 287 (2002); Shibata et al., Journal of Catalysis 222, 386, (2004); Kameoka et al., Physical Chemistry Chemical Physics (PCCP) 2, 367 (2000); and Meunier et al., Journal of Catalysis, 187, 493 (1999).
It was shown in these reports that NOx and hydrocarbons react on silver/alumina catalysts to form several transient nitrogen-containing intermediates. Nitrogen can be formed from the nitrogen-containing intermediates in at least three ways: 1. from the reaction of the nitrogen-containing intermediates with one other; 2. from the reaction of the nitrogen-containing intermediates with NOx, or 3. from the decomposition of the nitrogen-containing intermediates. The nitrogen-containing intermediates can be, for example, ammonia, organic nitrates, nitroso compounds, cyanates, isocyanates, etc. Most of these nitrogen-containing intermediates are toxic.
The hydrocarbon used for NOx reduction can undergo partial oxidation on the silver surfaces to form carbon monoxide. Carbon monoxide is a harmful pollutant. There is a need for a catalyst that selectively converts NOx to nitrogen without forming secondary emissions such as the previously described nitrogen-containing intermediates and/or carbon monoxide.
Researchers at Abo Akademi University in Finland (Eränen et al. Journal of Catalysis, 219, 25 (2003)) reported that 90% of the NO in an exhaust stream was converted to N2 at 450° C. with a 2% silver/alumina catalyst using octane as a reductant. The mean conversion of NO in the temperature range of 300-600° C. was 66%. A considerable amount of carbon monoxide was generated, however.
Eränen et al. were able to oxidize the carbon monoxide to carbon dioxide by contacting the gas stream with a commercial oxidation catalyst placed after the silver/alumina catalyst. The oxidation catalyst converted 100% of the carbon monoxide in the temperature window from 150 to 6000° C.
Surprisingly, the NO conversion to N2 at 450° C. declined from 90% with the silver/alumina catalyst alone to 45% with the combination of the silver/alumina catalyst and the commercial oxidation catalyst. The average NO conversion in the temperature interval of 300-600° C. fell from 66% with the silver/alumina catalyst to 32% with the combination of the silver/alumina catalyst and the commercial oxidation catalyst. Conversion of nitrogen to nitrogen oxides is not thermodynamically favorable in the 300-600° C. temperature range. Eränen et al. posed the question of how nitrogen can seemingly disappear from the exhaust gas with the combination of the silver/alumina catalyst and the commercial oxidation catalyst.
Eränen et al. performed an additional series of experiments in which the silver/alumina catalyst and the commercial oxidation catalyst were placed at various distances from one another. The conversion of NO to N2 increased as the distance between the two catalysts increased. The best NO to N2 conversion was achieved with a gap of 33 mm between the two catalysts.
The NO to N2 conversion at 450° C. with a 33 mm gap was about 88%. By comparison, when the two catalysts were physically mixed, completely eliminating the gap, the NO to N2 conversion was less than 10% over the entire temperature range of 150-600° C. Eränen et al. concluded that nitrogen-containing intermediates formed on the silver/alumina catalyst react with one another in the gas phase. They believed that providing a large gap between the two catalysts allows time for the intermediates to react with each other.
Although one could design a system with a large gap between the two catalysts, the resulting apparatus would be bulky. Further, there is no guarantee that the size of the gap that is required would remain constant from system to system.
Miyadera and Yoshida (U.S. Pat. Nos. 6,057,259 and 6,284,211) describe an exhaust gas cleaner and a method for removing NOx from an exhaust gas by bringing the exhaust gas into contact with the exhaust gas cleaner in the presence of oxygen-containing organic compounds. The exhaust gas cleaner contains a first catalyst containing silver and a second catalyst containing tungsten oxide or vanadium oxide. The second catalyst is capable of reducing the nitrogen oxides with ammonia generated by the first catalyst.
Vanadium oxide is toxic and is difficult to dispose. There is a need for a non-hazardous catalyst for selectively reducing NOx in the presence of reducing agents.
There is a need for a catalyst and a method for the selective reduction of NOx using hydrocarbons or alcohols as reducing agents, where the catalyst is able to convert NOx selectively to N2 without forming secondary emissions such as carbon monoxide and/or toxic nitrogen-containing intermediates. There is also a need for a catalyst and a method for selectively reducing NOx with hydrocarbons or alcohols where the catalyst system does not rely on a gap between two catalysts, because the gap could lead to non-reproducibility or to a bulky system that is unsuitable for applications that are limited in terms of space. There is also a need for a catalyst and a method for selectively reducing NOx with hydrocarbons or alcohols where the catalyst system does not rely on a catalyst that contains hazardous components such as vanadium.